The way teachers see it, today's classroom environment often deserves a D - as in disrespectful, distracting and disheartening enough to drive many of them away.

Most teachers in middle and high schools say misbehavior by a handful of children is such a disruptive, pervasive force that a majority of students suffer for it, a study said.

Although schools have become better at responding to serious offenses, such as guns and drugs on campus, the cumulative problem of routine unruly behavior is undermining academic achievement, says the report by Public Agenda, a nonpartisan research group.Most teachers say they contend with students who disrespect them, cheat, show up late and harass others.

"If you start totaling up the hours that teachers could be teaching and students could be learning, it's just staggering," said Public Agenda president Ruth Wooden. "We've got lots of programs on things like accountability and testing and parent involvement, but we haven't been nearly as successful at this daily distraction that takes teachers off-task."

More than three in four teachers said they could do their job more effectively if not for discipline problems; more than eight in 10 said most students suffer because of a few troublemakers. Nevertheless, the time spent on crowd control is not the kind of problem that generates public attention or outrage among policy-makers, the study said.

It is a struggle, however, that gives teachers second thoughts about their choice of careers.

About a third of teachers have considered quitting because of student indiscipline, according to the study, which surveyed both teachers and parents. About one-third also said they know colleagues who quit or were forced out over student misbehavior.

Tina Dove, a teacher of six years, used to head home drained with frustration after dealing with disruptive students and hearing about their often troubled conditions at home. A no-nonsense disciplinarian, as she put it, Dove still lost patience with the classroom struggle and took time off this year. At 32, she's hoping to return to teaching.

"If you have a child in your classroom who is difficult to work with, and they are setting a tone, you can have anything from a five-minute distraction to the loss of half a class period," said Dove, who lives in Alexandria, Va. "If you try to deal with that child in a way that's going to have the least impact on everyone else ... that can take up an amazingly large period of your class. Before you know what happened, you're behind."

The challenge, the study said, is otherwise complicated: Education colleges don't prepare teachers to deal with rowdy students; children in special education are treated too lightly even when their misbehavior has nothing to do with their disabilities; schools back down from discipline when parents threaten lawsuits.

"Parents are much more willing than they were 20 years ago to lawyer up and fight," said Julie Underwood, general counsel for the National School Boards Association.

The biggest cause of student behavior problems, according to both teachers and parents, is that too many parents fail to teach their kids discipline. Meanwhile, schools have less time to teach social skills or to free kids for recess or exercise, said Jerald Newberry, who directs the health and safety division at the National Education Association, the nation's largest teachers union.

Teachers say proposed solutions that might be effective include holding parents more accountable for their child's behavior and strictly enforcing rules on minor offenses to set the right tone for students.

The study was based on a mail survey of 725 public middle and high school teachers and a telephone survey of 600 parents of public school students in grades five through 12. Both had had a margin of sampling error of plus or minus 4 percentage points. The study was financed by Common Good, a bipartisan legal reform coalition.